Piston for a hydraulic piston cylinder unit

ABSTRACT

It is frequently important for the piston of a hydraulic piston-and-cylinder assembly, in particular the brake piston of a disc brake, which is slidably arranged within a cylinder bore to ensure a true-to-size mounting position of the piston inside the cylinder bore. Especially in the case of brake pistons of a disc brake, this determines the efficiency of provisions on the brake piston for reducing the occurrence of brake noises. According to the present invention, a key-type accommodation is provided on the piston which permits engagement by a matingly configured mounting means or mounting tool and, thus, renders possible the defined positioning of the piston within a cylinder bore.

TECHNICAL FIELD

The present invention relates to a piston for a hydraulicpiston-and-cylinder assembly, in particular for a brake piston of a discbrake.

BACKGROUND OF THE INVENTION

Generic pistons of this type, when used as brake pistons in disc brakes,are among others also responsible for the production and/or transmissionof undesirable brake noises. To reduce such brake noises, the brakepistons normally have an appropriate configuration. German patentapplication No. 38 33 552, for example, discloses a spot-type disc brakewhich has a generic bowl-shaped brake piston. The bowl-shaped brakepiston bears with one front side against the back side of a brakelining, and the bowl bottom faces the hydraulic pressure chamber in thebrake housing. On its end surface facing the friction lining, the brakepiston has a step, with the result of a sickle-shaped, rather than anannular, abutment surface between the brake piston and the frictionlining back plate. Such a sickle-shaped abutment between the brakepiston and the brake lining produces a slight shift in the resultingpressure point between the brake piston and the brake lining and resultsin a positive reduction of brake noises. The result of such amodification of the brake piston for noise reduction purposes is therequirement of a true-to-size orientation or positioning of the brakepiston within the brake cylinder, in particular in order to avoid anundesirable inclined positioning of the piston-side brake lining. Such apositioning is especially related to the exact angular position of thepiston in relation to a rotation about the cylinder bore axis. In priorart brake piston designs, it is only to an insufficient extent notpossible to position the brake piston inside the brake cylinder withsufficient accuracy to achieve the noise reduction results attained bythe present invention.

An object of the present invention is to provide a piston for ahydraulic piston-and-cylinder assembly which is easy to mount and can beplaced in a defined position within a cylinder bore.

Preferably, the piston includes a key-type tool accommodation suited forengagement by a mounting means of a corresponding configuration andintended for the defined positioning of the piston inside a cylinderbore. Thus, the piston can be arranged in a true-to-size fashion withina cylinder bore during the installation of the piston in the cylinderbore. More particularly, this ensures precisely maintaining the angularposition with respect to a rotation about the cylinder bore axis.

According to a favorable design variation, the mounting means, which isparticularly configured as a mating mounting tool, and thecorrespondingly configured accommodation in the piston areform-lockingly engaged similar to a male-female plug assembly. Thispermits applying the mounting means or tool only in one single positionon the accommodation in the piston, thereby allowing a defined alignmentof the piston inside the cylinder bore.

It is especially favorable to configure the above-mentionedaccommodation on one of the front surfaces of the cylindrical piston or,with a substantially bowl-shaped configuration of the piston (as is thecase particularly with brake pistons), on the inner surface of the brakepiston bowl.

A design variation of the key-type accommodation which is particularlyeasy to achieve, results from the provision of a corresponding recess orprojection on the piston. A recess or projection of this type permits,as has already been mentioned, the key-type defined engagement orapplication of an associated mounting means and has a correspondinglyshaped contour (as can be taken from the drawings in particular).

Preferably, at least one of the end surfaces of the cylindrical pistonhas an inclined positioning, at least in sections, with respect to aplane which extends vertically to the direction of piston displacement.This achieves for a brake piston in a normal brake operation an inclinedpositioning of the piston inside the cylinder bore which is favorable toprevent brake noises. Favorably, a step which extends in a secant'sdirection of the piston takes its course through the chamfered surface.In its depth in the direction of piston displacement, this step is sodimensioned that it can be used for the exact positioning of the pistonduring installation by making use of optical means (for example, Laser,monitoring by means of pictures, video, etc.) for detection of theposition of the piston. In a combination with the described positioningdevice for the brake piston, this permits taking effect on thedevelopment of brake noises.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 to 3 show two partly cross-sectional views of a bowl-shapedbrake piston with a key-type accommodation in the interior of thepiston.

FIGS. 4 to 5 show a top view of a bowl-shaped brake piston with anothervariation of the design of the accommodation in the interior of thepiston.

FIG. 6 shows two views of a bowl-shaped brake piston with a chamferedend surface portion and a step on the end surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The variations of brake pistons 1 shown in the embodiments of FIGS. 1 to6 are all suitable for use in a disc brake (not shown) especially forautomotive vehicles. Each of the brake pistons 1 is axially slidablysupported in a cylinder bore of the brake housing. At its end facing thehydraulic pressure chamber within the brake housing, each cylindricalbrake piston 1 has a bowl bottom 2 and a substantially annular endsurface 3 on the opposite side of the brake piston for abutment of thebrake piston 1 with a (non-illustrated) back plate of a brake lining. Indetail, the brake piston 1, among others, influences the noise behaviorof an associated automotive vehicle brake and, to this end, isfrequently furnished with noise-reducing design features. These featuresespecially include measures on the end surface 3 which have an effect onthe contact surface between the non-illustrated brake lining and brakepiston 1. In the FIG. 1 embodiment, for example, the design of a step 4at end surface 3 is shown which causes a sickle-shaped abutment 5 of thebrake piston 1 on the brake lining. In addition, step 4 at end surface 3causes an inclined position of the brake piston 1 within the cylinderbore which is favorable with respect to noise reduction. The amount ofthe inclined position of the brake piston 1 and the sickle-shapedabutment 5 is greatly dependent on a true-to-size positioning of thebrake piston 1 inside the cylinder bore (not shown). It is in particularsignificant in which angular position, with respect to a rotation aboutthe cylinder bore axis 6, the brake piston 1 is positioned within thecylinder bore during the installation. On the basis of a non-occurringrotation of the brake piston within the cylinder bore during brakeoperation it is already during the precisely positioned assembly of thebrake piston 1 that major influence is taken on the efficiency ofnoise-reducing provisions on the brake piston 1. Consequently, theefficiency of the sickle-shaped abutment 5 of the inclined position ofthe piston is determined during the exactly positioned installation ofthe brake piston.

To this end, a key-type accommodation is provided on the brake piston 1into which a correspondingly configured mounting means, in particular anassociated mounting tool, may engage in order to position the brakepiston 1 inside the cylinder bore. With an appropriate dimensioning ofthe step 4 at the end surface 3, a mounting tool of this type may easilymake catch at the said step 4 and place the brake piston 1 duringmounting in a precisely defined angular position within the cylinderbore. When the step 4 has an especially flat design, the applicationsurface for a tool is insufficient, what requires the provision of anadditional accommodation for a mounting means.

Apart from the design variations illustrated in the Figures, it ispossible in an especially advantageous manner to provide a groove-shapedindentation in the annular end surface 3 of the piston 1 and to use thisindentation for the rotationally exact positioning of the piston duringinstallation. A corresponding mounting means may engage into such agroove-shaped indentation and perform the exactly positionedinstallation, and the position of the piston can be determined andcontrolled especially by use of optical auxiliary means or sensors (forexample, Laser, monitoring by means of pictures, video, etc.).

Three indentations 7 in a semicircle are shaped into the bowl bottom 2in the piston interior according to FIG. 1. The indentations 7 may besimple circular holes which are provided in the bowl bottom 2 alreadyduring manufacture of the entire brake piston 1. The special arrangementof the indentations 7 in the bowl bottom 2 permits the definedapplication of a matingly configured mounting tool with associatedkey-type projections which form-locking engage into the associatedindentations. The mounting tool can only be applied in one singleposition on the brake piston 1 which prevents the risk of faultyassembly. Accordingly, the brake piston 1 can be inserted into theassociated cylinder bore with a defined angular position with respect toa rotation about the cylinder bore axis 6.

Further variations of a key-type accommodation in the interior of abowl-shaped brake piston 1 can be seen in FIGS. 2 to 6. According to apreferred aspect of the invention, the brake piston 1 of the FIGS. 2 to6 embodiments includes an inclined portion 8 on its end surface 3 whichfaces the brake lining (not shown). Arranging such an inclination 8 onthe end surface 3, compared to a step 4 in FIG. 1, has advantagesespecially with regard to tension. The tension peaks which occur in thearea of step 4 in an arrangement as disclosed in FIG. 1 are eliminatedin an arrangement according to FIG. 2 because there is an almostcontinuous transition between the sickle-shaped abutment surface 5 andthe inclined portion 8. The arrangement of an inclined portion 8 on thebrake piston end surface 3 thus proves particularly favorable in brakepiston designs made of plastics or aluminum. The key-type toolaccommodation in FIG. 2 is a semicircular recess 9 in the bowl bottom 2.

All other Figures show two design variations of a slot-shaped recess(FIG. 3, FIG. 4) and a T-shaped contour of the recess (FIG. 5), eachinside the bowl bottom 2. It is not absolutely necessary to arrangeslot-shaped or groove-shaped recesses of this type in the interior ofthe bowl-shaped piston. Accordingly, they may of course also be providedat a different location, especially on the end surface 3 which is closeto the brake lining. The different design variations of the toolaccommodation 7, 9, 10, 11, 12 have a similar functioning and permit theprecise application of the mounting tool with a correspondingly shapedkey-type attachment in a defined angular position. The configuration ofthe key-type accommodation of the brake piston is, of course, notlimited to the design variations shown and permits applying theprinciple of the present invention to any desired contour of a workpieceholding fixture which renders possible a defined engagement by anassociated, fitting mounting means and, thus, a defined positioning ofthe brake piston 1 within the cylinder bore.

The embodiment of FIG. 6 shows a brake piston 1 of the present inventionwhich includes the above-mentioned chamfered surface portion 8 at itsend surface 3 which is close to the brake lining (not shown). Theinclination 8 is defined by a step 14 in the direction of pistondisplacement which extends in a secant's direction of the cylindricalbrake piston 1. The depth of the step 14 in the direction of pistondisplacement is so dimensioned that it is appropriate for theapplication of a mating auxiliary mounting means for the rotationallyexact positioning of the piston. Similar to the design variation shownin FIG. 6, the step 14 in the secant's direction of the piston may takeits course through the chamfered surface portion 8 so that inclinedpartial surfaces extend on either side of the step 14. In addition, itis provided to determine and control the piston position duringinstallation by making use of optical means or sensors (for example,Laser, monitoring by means of pictures, video, etc.). This permits anautomatic piston installation with a rotationally exact positioningirrespective of the feeding of the individual brake pistons to themounting device.

Further, the arrangement of a positioning device is not bound to beingemployed on brake pistons for disc brakes. Rather, it can be applied toany pistons of a hydraulic piston-and cylinder assembly where aprecisely positioned installation of the piston within a cylinder boreis required. It is of great significance that the mounting means and themating accommodation in the piston form-lockingly engage each other, forexample, similar to a male-female plug assembly. It is of no importance,however, whether the accommodation 7, 9, 10, 11, 12 in the piston 1exhibits a male or a female configuration.

What is claimed is:
 1. Piston for a hydraulic piston-and-cylinderassembly, comprising: a piston body having a bowl shaped portion andhaving a key accommodation portion for the defined positioning of thepiston body, wherein the accommodation portion is shaped as at least onerecess or projection into the bowl shaped portion.
 2. Piston for ahydraulic piston-and-cylinder assembly, comprising: a piston body havinga bowl-type configuration, wherein said piston body includes anaccommodation portion for the defined positioning of the piston, whereinat least one end surface of the cylindrical piston has an inclinedsurface portion with an indentation for engagement of an associatedmounting means.
 3. Piston for a hydraulic piston-and-cylinder assembly,comprising: a cylindrical piston body having a bowl-type configurationand an accommodation portion for the defined positioning of the piston,wherein the piston body includes at least one end surface generallyoriented 90°, with respect to the side walls of the cylindrical pistonbody, and wherein said at least one end surface has an inclined surfaceportion on which is provided a step that extends in a secant's directionof the cylindrical piston.
 4. Piston as claimed in claim 3, wherein thestep defines the inclined surface portion.